1. Field of the Invention
The present invention relates to display device particles that are used for a display device, a process for producing the display device particles, as well as a repeatedly rewritable image-display medium and an image-forming device each using the same.
2. Description of the Related Art
As repeatedly rewritable image-display media, display techniques proposed hitherto include a twisting ball display (display utilizing rotation of particles having two separate colors), electrophoresis, magnetophoresis, thermally rewritable media, liquid crystals having memorizing ability, and the like. Such display techniques are excellent in image memorizing ability, but they have entailed problems insofar that the display has not been able to produce paper-like white color at its surface, and density contrast of images has been low.
In order to solve the above-mentioned problems, a display technique using a toner has been proposed. In this technique, a conductive colored toner and white particles are used to exhibit contrast, and thereby display images. In more detail, the conductive colored toner and the white particles are sealed in a void between electrode substrates facing each other, and electric charges are applied on the conductive colored toner through a charge transporting layer deposited on an inner surface of the non-display side of the electrode substrate, such that the charge-introduced conductive colored toner can migrate, via an electric field applied across the two electrode substrates, toward the display side of the electrode substrate, positioned opposite the non-display side of the electrode substrate, and thereby reach, and adhere to, the inside of the display side of the electrode substrate (see Japan Hardcopy '99, pp. 249-252).
This display technique, in which the image-display medium is made entirely of solid materials, is excellent insofar that displays of white-color and displays of black-color can in principle be completely interchanged. This technique, however, also entails drawbacks insofar that some of the conductive colored toner particles do not contact the charge transporting layer deposited on the inner surface of the non-display side of the electrode substrate, and that other conductive colored toner particles are produced which are isolated from the other conductive colored toner particles. But of these types of conductive colored toner particles cannot migrate, via the electric field, since no electric charges are applied on the toner particles. As a result, toner particles appeared at random between the two electrode substrates, causing a problem of diminished contrast density.
As an image-display medium using particles and excellent in density contrast, an image-display medium has also been proposed which comprises a pair of substrates and uses plural kinds of particles that have mutually different colors and charging characteristics, and are sealed in a void between the pair of substrates to allow migration between the substrates via an applied electric field (see Japanese Patent Application Laid-Open (JP-A) No. 2001-312225). This proposed technique has produced high-grade whiteness and high density contrast. The particles used in this image-display medium have been able to produce, at an initial stage, excellent white density, black density and density contrast. However, when images have repeatedly been rewritten over a long term, image density has been reduced, thereby leading to a decrease in density contrast, or to a lack of image uniformity, and thus on occasions causing image unevenness.
Image density is controlled by an electrical charge amount of the particles. Attempts have been made to adjust the electrical charge amount of the particles using a charge controlling agent that is conventionally used for an electrophotographic toner (see, e.g., JP-A No. 2002-10771). Use of the conventionally used charge controlling agent provided improvement in the electrical charge amount of the particles. However, it is still unsatisfactory to obtain desired density contrast by adjusting the electrical charge amount.